Dashboard enhancements:
- Load user.csv with 132,999 US and Canada users
- Memory-efficient storage: Dict[int, str] (callsign only)
- Automatic callsign lookup in last_heard updates
- ~9.5 MB memory usage vs ~21 MB for full database
User database:
- Filtered to US and Canada only (54.4% reduction)
- File size: 7.3 MB (down from 14.8 MB)
- 132,999 entries (down from 291,968)
- O(1) lookup performance
Benefits:
- Dashboard now shows callsigns in Last Heard table
- No external API calls needed
- Fast in-memory lookups
- Reasonable memory footprint for target networks
